An LED is the electrical equivalent of a diode. As such, it is a current-driven device. Apply the correct amount of current to the LED to attain its specified light output in lumens per watt. Supply less than this and you do not get the full amount of light output that it is capable of delivering. For a single LED, this might not seem like a big deal; however, if many LEDs are in a series string and are not getting uniform current, the light output will vary and be noticeable.
Of course, you also have to overcome the LED's inherent forward voltage drop, which can vary depending on the type of LED you have and the end fixture configuration. For a white LED, this forward voltage drop is usually around 3.5V but can be slightly higher at elevated temperatures. As a result, depending the input power source, a wide range of conversion topologies will be needed. This can be further complicated by the different LED string configurations, which are series (LED forward voltage drop additive), parallel (LED current additive) or a series of parallel string (both LED forward voltage and current additive).
Now that we understand what an LED needs for correct operation, what can happen to negatively impact their operation and the operation of its IC driver circuit?
The answer is that a lot of things can adversely affect an LED's output, or even lead to its catastrophic failure. These events consist of overvoltage, usually occurring during an open LED event, or an overcurrent condition, which normally happens during short circuits or the re-plugging of an LED string. Of course, poor thermal environments can adversely affect an LED's useful life and so a good overall thermal design is critical, too.
Armed with the knowledge of what can harm or even kill the LED and its LED IC driver circuitry, a lighting-systems designer must select design techniques and components for LED IC driver circuits that best safeguard against these dangers.
Some of the more critical aspects are:
(1) Tightly regulated current delivery to the LED. The LED IC driver circuit is key, since it must take whatever the input power source is, which will vary widely due to the broad range of applications, and convert it to the required voltage and the best current levels for LED performance. Since overvoltage or current can negatively impact either LED life or its light output over time, the tighter they can be regulated the more robust the system can be. Therefore, having +/-5 percent voltage and current regulation is going to be beneficial for long and trouble free life.
(2) To protect from over-voltage situations, an LED driver circuit that can handle higher transient voltage conditions above normal operation is a prerequisite. A good example is an automotive environment where load dump might be 42V, or even higher.
(3) Protecting from thermal overstress is a little more challenging. Quite often the LED deployment fixture is small and compact with very little in the way of heat sinking – and probably no fans to provide air cooling. Therefore, most of the heat will have to be dealt with via conduction. Thus, good heat sinking must be taken into account at the design concept. Also, having a LED driver circuit that operates with very high conversion efficiency is a great help. If not obvious, the higher the conversion efficiency, the less heat is produced as part of the conversion power loss. Thus, LED IC drivers with low to mid-90 percent efficiencies will significantly aid good thermal design.
Another interesting way to help with thermals is to have an on-chip temp sensor in the LED IC driver such that if there were a system controller to monitor this temp signal, it could have the overall system throttle back the current so as to allow less heat generation. Of course, this would be at the cost of reduced light output, but this is better than complete system failure. Once the fault condition goes away, normal operation could resume.
Finally, having these same practical approaches as discussed for the LED are also, by extension, applicable to the LED IC driver circuit as well. So having them incorporated in the overall system is a very good idea.
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